An example of the type of apparatus to which the present invention relates is an electricity rate meter, as described in German patent No. 24 46 602. An electricity rate meter is used for the continuous recording and display of the electrical energy consumption. These electrical energy consumption values are averaged over constant time intervals. Such apparatus is frequently operated with three-phase power supplied by a three-phase power supply unit. In such apparatus, pulses are generated according to the measured electrical power, and these pulses are stored for a specified period of time. The pulses are calculated from the integral of the measured electrical power versus time and are thus a measure of the consumed electrical energy drawn from the power supply. Pulses of constant frequency, which are derived from the frequency of the power supply, are needed as a time base.
Peak demand detectors also have a similar method of operation as described in German Patent No. 26 05 803. As with rate meters, peak demand detectors are supplied from an assigned power system and derive clock pulses for their time basis from this power supply.
An a.c. control voltage suitable for the formation of pulses can be derived from a three-phase power supply. In the case of a four-wire configuration, an a.c. control voltage is usually obtained from a phase voltage, that is, from the voltage between a phase line and the neutral line. In a three-wire configuration a line to line voltage serves this purpose. As a result, if the phase that is used for the a.c. control voltage fails, clock pulses will cease to occur even though the apparatus is still sufficiently supplied with current. Thus, the entire apparatus does not function properly during the failure of the respective phase voltage.
There is a need to maintain the operation of an apparatus driven by three-phase power, even when failure occurs in one or two phases of the three-phase power. In other words clock pulses need to be continuously generated as long as the apparatus remains in operation with a sufficient power supply.